To see if there is any relationship between blood pressure and childbearing, researchers examined data from a large health survey. The following table shows the data on systolic blood pressure (mm Hg) for random samples from two populations of women: women who had borne no children and women who had borne five or more children. The pooled standard deviation from all eight groups was Spooled = 18 mm Hg.30

Carry out age adjustment, as directed, using the following reference distribution, which is the approximate distribution for U.S. women31:

(a) Calculate the age-adjusted mean blood pressure for women with no children.
(b) Calculate the age-adjusted mean blood pressure for women with five or more children.
(c) Calculate the difference between the values obtained in parts (a) and (b). Explain intuitively why the result is smaller than the unadjusted difference of 127 - 121 = 6 mgHg.
(d) Calculate the standard error of the value calculated in part (a).
(e) Calculate the standard error of the value calculated in part (c).

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Five or more children No children Mean Mean blood No. of blood No. of Age pressure women pressure women 18-24 113 25-34 118 35-44 125 45-54 134 18-54 121 230 110 105 123 568 114 116 124 138 127 82 127 124 340 Age Relative frequency 18-24 25-34 35-44 45-54 0.17 0.29 0.31 0.23